1. Field of the Invention
This invention relates to a low temperature firing glass-ceramic substrate permitting co-firing with a low-resistance conductor such as Au, Ag or Cu, especially a glass-ceramic substrate having a low dielectric constant, a small dielectric loss and high strength and also to a production process thereof.
2. Description of the Related Art
Ceramic substrates have been used as high-density wiring substrates because they easily allow high-degree multilayer wiring by the green sheet lamination process. Although aluimna is most common as a ceramic material for them, it is accompanied by problems that (1) for its firing temperature as high as about 1,600.degree. C., high-resistance W or Mo has to be chosen as a wiring material for internal layers to be formed by co-firing and (2) for its relative dielectric constant as high as about 10, a reduction in dielectric constant has to be achieved desirably from the viewpoint of achieving high-speed transmission.
On the other hand, glass-ceramic materials, which are composite materials of glass of relatively low softening point and ceramics, have been developed as multilayer wiring substrate materials capable of overcoming the above-mentioned problems, because (1) they can be fired at 900 to 1,000.degree. C. and can hence choose a low-resistance conductor such as Au, Ag or Cu as a conductor metal for internal layers and (2) they can attain a lower dielectric constant by using glass of low dielectric constant.
Nonetheless, low-dielectric-constant materials have a problem in reliability as the dielectric constant and strength of a material are generally in a substantially proportional relationship. In addition to this trade-off relationship between dielectric constant and material strength, there is an increasing demand for a reduction in the dielectric loss of a substrate material to meet the requirement for lower transmission losses in high frequency circuits employed in a communication network.